Sensors of various varieties have been used for decades to produce electrical signals related to various physical quantities. Thermistors for producing signals related to temperatures and accelerometers for producing signals related to acceleration are simple and familiar examples. The present invention finds application with all types of sensors but, for convenience of illustration, is illustrated with reference to accelerometers.
Accelerometers have a multitude of applications. One is in the modal testing of large physical structures, such as aircraft wings. In such applications, several hundred accelerometers may be mounted to different positions on the wing's surface and the structure then mechanically excited by shakers. The accelerometers produce data signals related to the motion of the wing at each position This data is collected by data acquisition equipment and can later be analyzed to determine the modes of oscillation of the structure.
Modal analysis of structures requires that each transducer used in the test be identified and correlated to a particular position on the structure. In a test involving hundreds of these instruments, the data acquisition equipment is provided with hundreds of indistinguishable accelerometer output cables. (Accelerometer output cables typically have only two wires, even if the accelerometer includes active electronics which must also be powered from the cable. This dual use of the cables is routinely accomplished by "ICP" techniques wherein power is provided to the sensor as a constant current signal and the sensor output signal takes the form of voltage modulation on the power signal.) If even a single one of these accelerometer cables is misidentified or connected to an incorrect port on the data acquisition equipment, the entire modal analysis experiment may be ruined. Consequently, it is critical that each cable be precisely traced through the bundle of cables and connected to the input port of the equipment that is expecting its signal. This is a tedious and error prone exercise.
It is an object of the present invention to simplify this task.
It is a further object of the present invention to improve data acquisition integrity in complex test and measurement applications by eliminating a potential source of human error.
It is a more particular object of the present invention to provide a sensor that can identify itself.
It is still another object of the present invention to provide a self-identifying sensor that can send identification data to the associated data acquisition equipment over existing wiring.
It is a more general object of the present invention to provide a test and measurement system that can exchange digital attribute signals between sensors and the data acquisition equipment over existing wiring.
According to one embodiment of the present invention, a sensor is provided with means for producing a digital "signature" signal uniquely identifying itself. This signal is relayed to the data acquisition apparatus over the same lines as are used for conveying the sensor data. Thus, the cables connecting the instruments to the data acquisition apparatus need not be individually traced. The signature data provided over the cable identifies the sensor from which the cable originated. Consequently, the data acquisition equipment knows the source of each data signal and can utilize the data properly. No longer must each cable be routed to the single input port that is expecting it. Instead, the cables can be connected randomly to the equipment. In a typical installation, the sensor data acquired by the test equipment is stored in association with the sensor identification data so that it can be recalled and processed as necessary to perform the desired analysis.
The illustrated self-identifying accelerometer comprises a piezoelectric transducer and an associated integrated circuit. Many integrated circuits today push the limits of both speed and the number of pins to the outside world. The circuitry of the present invention does neither. The circuit operates in the one hertz to one kilohertz range and has three pins: TRANSDUCER INPUT, GROUND and SUPPLY. The circuit's output signal is provided on the SUPPLY line and takes the form of voltage modulation of a constant current power signal carried to the circuit by this line.
When power is first applied to the illustrated accelerometer, the integrated circuit waits approximately 10 milliseconds to permit transients from the power supply to cease. The circuit then begins its signature phase of operation. During this signature phase, the circuit transmits over the power line a 36-bit serial signature data stream which uniquely identifies the sensor and may also include information about its operation (i.e. gain settings, ambient temperature, etc.). After the data stream has been transmitted, the integrated circuit operates as a buffer amplifier to modulate the sensor output signal onto the power signal. The circuit continues to operate as a buffer amplifier until the power signal is interrupted. When power is subsequently restored, the circuit again begins with its signature phase of operation.
The foregoing and additional objects, features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.